1. Technical Field
The present invention relates to a variable resistance nonvolatile memory device and a method for writing into the same. More specifically, the present invention relates to a variable resistance nonvolatile memory device which performs a verify write operation, and a method for writing into the same.
2. Description of the Related Art
Unexamined Japanese Patent Publication No. 2012-27962 discloses a technique concerning a NAND flash memory. A nonvolatile memory device described in Unexamined Japanese Patent Publication No. 2012-27962 has a memory cell array formed by arranging in a matrix shape a plurality of nonvolatile memory cells where a threshold during data-erasing is included in a first threshold distribution and a threshold during data-writing is included in a second threshold distribution. During data-erasing, an erase voltage is applied to a nonvolatile memory cell from which data is to be erased, and a threshold during data-erasing is shifted so as to be included in the first threshold distribution. The shift of the threshold of the nonvolatile memory cell from which data is to be erased into the first threshold distribution is verified by an erase verify operation with an erase verify level taken as an index. During data-writing, a writing voltage is applied to a nonvolatile memory cell in which data is to be written, and a threshold during data-writing is shifted so as to be included in the second threshold distribution. The shift of the threshold of the nonvolatile memory cell in which data is to be written into the second threshold distribution is verified by a write verify operation with a write verify level taken as an index. Then, the erase verify level and the write verify level are adaptively changed based on at least one of a state of the erase operation and a state of the write operation.
PCT International Publication Nos. WO2008/149484 and WO2009/050833 each disclose a variable resistance nonvolatile memory element using tantalum oxide (TaO) for a variable resistance layer.